HK10 - Stereo KT88 / 6550 Push-Pull Amplifier

the thermionic watercooler

Postby EWBrown » Sun Dec 21, 2008 8:35 am

I worked at DEC for about 17 years, and I met and talked with Ken Olsen (AKA KO to his employees) and he was a really nice guy, that rare CEO who would still talk with, and listen to "the little guy" . KO lived a very clean life, never consumed any "adult beverages", but his brother Stan, who had been "number two" in the DEC chain of command, he made W. C. Fields look like Carrie Nation in comparison... ;) (lol) (b) (wine) (d) :P :))

I well remember YDI, in Needham, it was one of my main parts sources back in the late 60s and early 70s. Its good to see that it is still around and hasn't just turned into yet another yuppified I-pod, cell phone and computer store.

Definitely that amp's PSU needs some heftier rectifier diodes, the 4007s will work, but for how long ?....

/ed B
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Postby EWBrown » Sun Dec 21, 2008 8:44 am

That Triode ELectronics PA-060 is quite conservatively rated, as opposed to the "just strong enough" original Dynaco predecessor.

Even the Triode 400-0-400VAC PA728 (?) MKIII power trannie is a lot stronger than the original, as well.

Both will run less hot than the vintage power iron, and have about 50% more reserve current.

/ed B
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Postby Brik » Sun Dec 21, 2008 10:09 am

EWBrown wrote:Definitely that amp's PSU needs some heftier rectfier diodes, the 4007s will work, but for how long ?....


I still have 97 of them left from the pack Yellow_Light_Colorz_PDT_01 Yellow_Light_Colorz_PDT_02 Yellow_Light_Colorz_PDT_03

But seriously, since Ed Brown, Ty, Don, and Soren all suggested that the rectification be upgraded, I ordered a bunch of heavier duty diodes from Mouser:

UF5408-E3/54 - Ultra Fast Recovery Rectifiers Vr/1000V Io/3A
http://www.mouser.com/Search/ProductDet ... -UF5408-E3

BY255P-E3/54 - Standard Rectifiers 3.0 Amp 1300 Volt
http://www.mouser.com/Search/ProductDet ... -BY255P-E3

I am not sure if my tin-ears can tell the difference between the two types, but it is a lot cheaper than comparing two types of $20/foot speaker wires.

I used to work for Digital Equipment also in Littleton, MA. during the 80s, and have lots of fond memories - huge rats who lived in the obsolete PDP-11/70 cabinet at The Mill, "discontinued equipment" in The Pond, etc...Yellow_Light_Colorz_PDT_17

The Edcor XPWR071 can probably power a 90-100W/channel KT88 PPP stereo amplifier like this one (with an additional heater transformer):
Image

As Ty said, whether one can safely lift the stereo version of such an amplifier in one chassis remains to be seen. :o

/b
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Postby EWBrown » Mon Dec 22, 2008 6:35 am

You're best off keeping it as two monoblocks, as trying to put twice of all of that iron and circuitry on a single chassis would be a daunting task, not to mention becoming a potential hernia and bad back generator... (Trust me, I know from experience, having fractured my L4 and L5 in 1989).

When it comes to rectifiers, bigger is definitely better.

IIRC, the VTL amps use the "BY" series of heavy duty "standard" rectifiers.

/ed B
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Postby dhuebert » Mon Dec 22, 2008 5:21 pm

Jeez, I am totally loving that schematic! It will definitely go in my schematic archive.

Don
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Postby Brik » Mon Dec 22, 2008 7:34 pm

Hi Don,

The PPP KT-88 schematic came from a kit tube amplifier site:
http://translate.google.com/translate?h ... html#hk-10

There are single-ended and push-pull 300B and 2A3 based amplifiers also.

I have been doing some heavy reading Yellow_Light_Colorz_PDT_03 on PPP tube amplifiers such as Convergent Audio Technology JL2:

http://www.soundstage.com/revequip/cat_ ... re_mk2.htm

Oh, my aching back
"....The JL2 Signature Mk 2 comes double packed in two extra-thick cardboard boxes, which make the fact that it's delivered by a freight company, not Fedex or UPS, all the more understandable. Lifting the amp out of those boxes is a two-person job that I stupidly did myself. It's an awkward heave, mostly because it's difficult to determine how to balance the amp once you have it off the ground. Don't overanalyze it; brute strength is the only thing that will get the job done...."
:o

Image

I think I'll stick with a ST-35 or a ST-70...Yellow_Light_Colorz_PDT_20

/b
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Postby Blair » Tue Dec 23, 2008 8:45 am

Hey Brik,

Do you have access to the schematic for the driver stage of that amp?

Or anyone got the front end schematic for the Cary six pack? I like the thought of using one of Shannon's board kits to drive my PPP amps, but still exploring options, plus I'm not sure how to wire the bias curcuit with multiple tubes.

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Postby Brik » Tue Dec 23, 2008 10:44 am

Hi,

No, I don't have the schematic for the CAT JL2.
Looking at the valve complement, I think it uses a 6922 in SRPP to drive the paralleled quad of 6550/KT88s.

It would have to be able to drive 100V-pp at perhaps as low as ~20K ohms (~100K ohms of control-grid-to-ground per tube).


/b
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herniator part II

Postby EWBrown » Tue Dec 23, 2008 11:22 am

Looks like that big "CAT" should be dubbed "The Herniator". I had given that name to a yet to be completed amp project, but that CAT makes mine look like a ziplock bag of feathers, in comparison :o


/ed B
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Postby EWBrown » Tue Dec 23, 2008 11:53 am

DEC in Littleton.... Were you there when the "great water UZI escapade" occured ?


For those outside of DEC, there was one day when a bunch of guys showed up at work, carrying the powered water guns, which looked very much like real UZIs, and the security guards panicked, they pulled a fire alarm to get everybody out of the building, then the local or Mass sate police raided the place.

No, I wasn't a participant, only heard about it over the DEC "notesfiles" intranet system. My 17 years of DEC were al in NH, between Salem, Hudson, Merrimack and Nashua.

Though I do remember hearing and reading a lot of the legendary "Mill Rat" stories, and the "shark in the pond" and lots of drowned obsolete gear in "the pond", and "the great chad blizzard" which also took place at "the mill".

And then there was the DEC helicopter which made an "emergency landing" in the Merrimack MK01 duck pond. I was around there for that event...


Those were the days.....


/ed B
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Postby Writer Frog » Wed Dec 24, 2008 11:07 am

deicide67 wrote:Do you have access to the schematic for the driver stage of that amp?

Or anyone got the front end schematic for the Cary six pack? I like the thought of using one of Shannon's board kits to drive my PPP amps, but still exploring options, plus I'm not sure how to wire the bias curcuit with multiple tubes.


The driver stage has to be able to put out 120V peak to peak at 33K ohms for a tri-paralleled, fixed-bias output stage in your case.
One way to do this is to use a 6SN7 SRPP like this:
http://www.triodeguy.com/Triodeguy%20PDF%20files/Triodeguy%20kt88aud.pdf

I don't know why this circuit needs the -260V supply.
Matt
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Postby Blair » Wed Dec 24, 2008 11:18 am

I have seen that front end and the -260v and all the parts seemed a bit more than I was wanting to tackle. Does anyone know what the voltage swing on the poseidon boards is? I'm not too limited on space, but I also want to keep it simple.

Thank you,

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Postby Writer Frog » Thu Dec 25, 2008 10:30 pm

deicide67 wrote:I have seen that front end and the -260v and all the parts seemed a bit more than I was wanting to tackle. Does anyone know what the voltage swing on the poseidon boards is? I'm not too limited on space, but I also want to keep it simple.

Thank you,

Blair


Hi Blair,

Happy Holidays to you!

What the voltage swing the Poseidon board can provide depends on the output load. You will have to design the output stage with a fixed bias scheme first to determine the output impedance that the Poseidon board would be driving.

If its ~100K then Poseidon should have no problem.

Matt
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Postby Blair » Fri Dec 26, 2008 1:57 am

Thanks! You too!

I was not planning to use a fixed bias on the power tubes. Each tube would have independent pots to adjust their bias. Am I reading your post wrong when I read "fixed bias"?

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Postby Ty_Bower » Fri Dec 26, 2008 7:16 am

"Fixed bias" means a negative voltage is applied to the grid of the tube to keep it biased. That voltage does not vary as the operating conditions of the tube change, hence it is "fixed". The cathode of the tube will be at ground potential (or very nearly so). It is customary and perhaps necessary that some provision be made so the negative voltage can be adjusted by the user to accommodate different tubes.

"Auto bias", or "cathode bias" means the bias is provided by means of a relatively large (100 to 1k ohm) resistor on the cathode of the tube. The drop across that resistor keeps the cathode above ground, biasing the tube. The grid of the tube will be at ground when idle. The However, the drop across the cathode resistor is not fixed - it can and will vary depending on the operating conditions of the tube. The higher the current through the tube, the greater the voltage drop across the cathode resistor. It is possible to build an user-adjustable auto bias circuit by using a small potentiometer on the cathode.
"It's a different experience; the noise occlusion, crisp, clear sound, and defined powerful bass. Strong bass does not corrupt the higher frequencies, giving a very different overall feel of the sound, one that is, in my opinion, quite unique."
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